The invention relates to a control system of battery health, which is used to measure the impedance and/or state of charge of the cells of a battery.
Batteries are used to ensure the supply of energy in important systems when breakdown has occurred. The safety equipment of energy generation and distribution, computers, equipment used in telecommunication, traffic-related security equipment, critical equipment in hospitals and in the back-up systems of lighting kinds of systems.
The condition of batteries is ensured by regular maintenance. The normal method of maintenance carried out 2-4 times a year is measuring the cell voltages and specific gravities in float charging situations. A load test is performed once in 1-3 years. In addition, the connections and internal corrosion of a battery are normally measured by impedance and conductance measurements. The difficulty in using Valve Regulated Lead Acid Batteries (VRLA-) is the high variation of cell voltage. Another difficulty is related to the drying-up of a cell causing a voltage increase and is compensated by sulfatization causing a voltage decrease. Therefore, the voltage measurement does not necessarily tell anything of battery health. Measuring impedance and conductance are normally used as methods of maintenance, although the correlation between these two and the battery health is not 100 percent. In addition, a high deviation even in cells of the same series and a strong temperature coefficient make it difficult to interpret the readings. In VRLA- batteries also, so-called thermal runaway exists, which is caused by the negative coefficient between gas voltage and temperature is a big problem, especially when old and cheap chargers lacking temperature compensation are used.
The load tests carried out on a regular basis make sure the battery is healthy. By comparing the increased impedance the test interval may be lengthened. The measurement of impedance and conductance by using the normal 4-wire-system (separate wires for current and voltage) requires the removing of the covers. Getting a good connection simultaneously in both the current and the voltage probe is difficult. Oxygenated surface may give incorrect readings if the probe does not penetrate lead oxide surface.
The U.S. patent document 5,214,385 recognizes the system of battery health, in which the state of charge is measured by using a separate power supply. In this case the measurement effects the total battery voltage and load voltage. The system is not used to measure impedance. The system used in the present invention according to the following description does not need a separate power supply, whereby the measurement does not effect the total battery voltage nor the load voltage. In addition the system of the present invention enables the measurement of state of charge alone or together with impedance measurement.
When the cell is charged electrical energy is used in a chemical reaction, in which lead sulfate becomes lead oxide and lead. When the cell is fully charged energy is used break down water. The gas forming on the electrodes increases cell voltage which in return decreases the float current. The total voltage of a battery is adjusted to a level (Stationary lead batteries typically 2.23 volts per cell) on which even the worst cell remains fully charged. In practice this means a continuous overcharging that shortens the lifetime of the cells.
One of the targets of the invention is to create a system that can be used in the follow-up of the battery health, especially used in the measurement of the internal corrosion based on impedance measurement and/or the measurement cells"" state of charge, which may be determined by measuring changes in cell voltage during shunt periods. Another target is to create a system that maintains the battery cells"" state of charge using a charge voltage slightly lower than the one used normally. This prevents the overcharging of the cells, which lengthens the lifetime of the cells. Yet another target is a low-cost way to carry out the invention.